Cryoablation systems are frequently used for treating tissue in a cardiac setting, either to cool the tissue sufficiently to stun it and allow cold mapping of the heart and/or confirmation of catheter position with respect to localized tissue lesions. Additionally, such systems may be used to apply a more intense cold to ablate an area of target tissue, for example, tissue that has been identified as propagating an aberrant electric current in cases of cardiac arrhythmia.
In general, when used for endovascular access to treat the cardiac wall, cryoablation catheters must meet fairly demanding limitations regarding their size, flexibility, and the factors of strength, electrical conductivity and the like. These constraints generally require that the catheter be no larger than several millimeters in diameter so as to pass through the vascular system of the patient to the heart. Thus, any electrodes and refrigerant passages must fit within a catheter body of small size.
A number of different fluids have been used for the refrigerant component of prior art cryotreatment catheters, such as a concentrated saline solution or other liquid of suitably low freezing point and viscosity, and of suitably high thermal conductivity and heat capacity, or a liquified gas such as liquid nitrogen. In all such constructions, the refrigerant must circulate through the catheter, thus necessitating multiple passages leading to the cooling area of the tip from the catheter handle. In some systems, a phase change refrigerant is used that travels through the body of the catheter at a relatively normal or ambient temperature and attains cooling only upon expansion within a chamber at the tip region. In some systems, pressurized gas travels through the body of the catheter and into a spray nozzle in a cooling chamber at the tip region, where cooling is achieved by the Joule-Thomson effect when the gas expands. Due to the size limitations on the device as a whole, however, the outlet holes of the spray nozzle or other fluid injection outlets are necessarily very small. Consequently, the outlet holes easily become clogged or blocked, which may lead to failure at the beginning of a cardiac procedure.
One cause of blockage in the cryoablation system may be ice formation that occurs at the first fluid injection at the beginning of a cardiac procedure. Ambient humidity may creep into the system, for example, the refrigerant injection lumen, refrigerant recovery lumen, and/or injection apertures. This ambient humidity may create pockets of moisture within the system that rapidly become frozen when the refrigerant is first injected into the system at the beginning of a new cardiac procedure, creating ice blockages within the system. When this occurs, the system must be thawed and purged before the procedure may take place, and this delay may have disastrous effects. The small holes of an injection nozzle are particularly susceptible to retaining moisture and becoming blocked with ice.
Moisture pockets generated by ambient humidity within the system must be addressed immediately before a cardiac procedure. New moisture pockets may develop in a short period of time, and therefore may be present at the beginning of a procedure even if the system was purged within a few hours of the procedure. However, presently known purging processes are inefficient or take too long, thus also contributing to a delay in urgent cardiac procedures.
Accordingly, a cryoablation system purging process is desired that is efficient and effective, and can be performed quickly immediately prior to the beginning of a cardiac procedure.